I'm an instrument tech that has no avionics experience, but a heap in
laboratory equipment that has high power valves and can say that the valves
will deteriorate with time and/or use, bit like a mobile phone battery where
you have standby and operating time, but you never know how well charged the
valve is to start with.

Some valves have the filament burn out, some get leaks into the vacuum
chamber, some have bits fall of internally like a suport wire in a light
bulb can and they short things out, but most just get older and weaker.

If you can talk to the tech that did the job on each of the second hand
units, you could get a general description of how clean the unit was inside
and what sort of output power level was there to start with, but most of
these things are just general pointers and the life expectancy of a valve is
the same as the length of a bit of string!

Considering that you can get two second hand ones for the price of a new one
and still have enought left over for several cartons of beer, I'd get a
second hand one and at least the first carton to celebrate?

Peter